The invention relates to a DC to DC converter for converting a DC input voltage into a plurality of DC output voltages.
Such a DC to DC converter is known from U.S. Pat. No. 4,628,426. In the converter described there, a DC input voltage is converted into a (pulse-shaped) AC signal by means of a full-bridge circuit comprising four switching elements, which AC signal is applied to the primary winding of a transformer. The transformer has two secondary windings. An AC signal produced by the first secondary winding is rectified and filtered to generate a first DC output voltage. The second secondary winding is connected in parallel to a resonant circuit capacitor which, together with the leakage inductance of the second secondary winding, forms a resonant circuit. A voltage dropping at the resonant circuit conductor is rectified and filtered to thus generate a second DC output voltage. The two DC output voltages are set to desired voltage values by respectively driving the switching elements of the full-bridge circuit and thus by adapting the AC signal produced by the full-bridge circuit. The first DC output voltage is set via an adaptation of the sampling ratio of the AC signal. The second DC output voltage is set via an adaptation of the frequency of the AC signal. The two DC output voltages can ideally be set independently of each other.
It is an object of the invention to provide the converter of the type defined in the opening paragraph so that the losses occurring when the converter is being operated are smallest possible.
The object is achieved in that the DC to DC converter mentioned in the opening paragraph includes
a circuit stage including switching elements for converting the DC input voltage into a first AC signal,
a first resonant circuit for converting the first AC signal into a second AC signal, which is used for forming a first DC output voltage,
a second resonant circuit for converting the first AC signal into a third AC signal, which is used for forming a second DC output voltage, and
a control circuit for setting various frequency ranges for the first AC signal.
This converter arrangement is particularly advantageous when the second DC output voltage is used for producing a DC standby supply voltage when the converter is being operated, while the first DC output voltage then becomes zero. In this operating mode there is only a small load on the second DC output voltage. Nevertheless, with the converter according to the invention a so-called ZVS operation (Zero Voltage Switching) of the switching elements of the circuit stage producing the first AC signal, which includes, for example, a half-bridge or a full-bridge circuit, is ensured (for a full understanding of the ZVS operation compare, for example, U.S. Pat. No. 5,808,879, which is not the case with said U.S. Pat. No. 4,628,426. With the converter according to the invention, the losses in the standby mode are reduced. The circuitry for realizing a normal operation and a standby mode is kept smallest possible. Normal operation and standby mode are set only via a suitable setting of the frequency range of the first AC signal and a respective drive of the switching elements of the said circuit stage. Different frequency ranges are then assigned to normal operation and standby mode. The frequency range assigned to the standby mode contains the resonant frequency of the second resonant circuit, which leads to the fact that the first DC output voltage is strongly reduced compared to the normal operation. A constant control of the output voltages is effected via a fine adjustment of the frequency in the respective frequency range.
Advantageous embodiments of the DC to DC converter are defined in the dependent claims. The invention also relates to a power supply that includes the DC to DC converter according to the invention.
Examples of embodiment of the invention will be described in more detail hereinafter with reference to the drawings, in which: